A typical prior art construction for an electrochemical cell is described in U.S. Pat. No. 6,080,503, the details of which are incorporated herein by reference. Electrochemical cells comprising polymer electrolyte membranes (PEMs) may be operated as fuel cells wherein a fuel and an oxidant are electrochemically converted at the cell electrodes to produce electrical power, or as electrolyzers wherein an external electrical current is passed between the cell electrodes, typically through water, resulting in generation of hydrogen and oxygen at the respective electrodes of the cell. FIG. 1 illustrates a typical design of a conventional electrochemical cell comprising a PEM, and a stack of such cells. Each cell comprises a membrane electrode assembly (MEA) 5 such as that illustrated in an exploded view in FIG. 1a. MEA 5 comprises an ion-permeable PEM layer 2 interposed between two electrode layers 1,3 which are typically porous and electrically conductive, and comprise an electrocatalyst at the interface with the adjacent PEM layer 2 for promoting the desired electrochemical reaction. The electrocatalyst generally defines the electrochemically active area of the cell. The MEA is typically consolidated as a bonded laminated assembly. In an individual cell 10, illustrated in an exploded view in FIG. 1b, an MEA is interposed between a pair of separator plates 11, 12, which are typically fluid impermeable and electrically conductive. The cell separator plates are typically manufactured from non-metals such as graphite or from metals, such as certain grades of steel or surface treated metals, or from electrically conductive plastic composite materials. Fluid flow spaces, such as passages or chambers, are provided between the plate and the adjacent electrode to facilitate access of reactants to the electrodes and removal of products. Such spaces may, for example, be provided by means of spacers between separator plates 11, 12 and corresponding electrodes 1, 3, or by provision of a mesh or porous fluid flow layer between separator plates 11, 12 and corresponding electrodes 1, 3. More commonly channels (not shown) are formed in the face of the separator plate facing the electrode. Separator plates comprising such channels are commonly referred to as fluid flow field plates. In conventional PEM cells, resilient gaskets or seals are typically provided between the faces of the MEA 5 and each separator plate 11, 12 around the perimeter to prevent leakage of fluid reactant and product streams.
Electrochemical cells with a ion-conductive PEM layer, hereinafter called PEM cells, are advantageously stacked to form a stack 100 (see FIG. 1d) comprising a plurality of cells disposed between a pair of end plates 17, 18. A compression mechanism (not shown) is typically employed to hold the cells tightly together, maintain good electrical contact between components and to compress the seals. In the embodiment illustrated in FIG. 1c, each cell 10 comprises a pair of separator plates 11, 12 in a configuration with two separator plates per MEA. Cooling spaces or layers may be provided between some or all of the adjacent pairs of separator plates in the stack assembly. The stack may comprise a cooling layer interposed between every few cells of the stack, rather than between each adjacent pair of cells.
A bipolar flow field plate is an assembly of two individual flow field plate components. The bipolar flow field plate services two adjacent fuel cells, serving as the anode for one fuel cell and the cathode for the other. This reduces the number of components that must be assembled to create a fuel cell stack, thus simplifying the construction of the fuel cell stack.
The cell elements described have openings 30 formed therein which, in the stacked assembly, align to form fluid manifolds for supply and exhaust of reactants and products and, if cooling spaces are provided, for a cooling medium. Again, resilient gaskets or seals are typically provided between the faces of the MEA 5 and each separator plate 11, 12 around the perimeter of these fluid manifold openings to prevent leakage and intermixing of fluid streams in the operating stack.
FIG. 2 is an enlarged schematic cross-sectional view of the individual fuel cell of FIG. 1b, showing schematically channels such as 13 and 14 in the individual flow field plates 11 and 12.
FIG. 3 is a schematic representation of a typical prior art bipolar flow field plate in which two of the individual flow field plates such as 11 and 12 are placed back to back and are held together by an adhesive 15. In the process of manufacturing the prior art bipolar plate 16 of FIG. 3, a graphite mat would be prepared, then impregnated, dried, washed, baked, stenciled with adhesive 15 on the backs of the plates, then the two plates would be pressed together and heat cured to form the bipolar plate 16.
As noted, the separator plates or flow field plates 11 and 12 may be constructed from graphite material.
Graphites are made up of layer planes of hexagonal arrays or networks of carbon atoms. These layer planes of hexagonally arranged carbon atoms are substantially flat and are oriented or ordered so as to be substantially parallel and equidistant to one another. The substantially flat, parallel equidistant sheets or layers of carbon atoms, usually referred to as graphene layers or basal planes, are linked or bonded together and groups thereof are arranged in crystallites. Highly ordered graphites consist of crystallites of considerable size: the crystallites being highly aligned or oriented with respect to each other and having well ordered carbon layers. In other words, highly ordered graphites have a high degree of preferred crystallite orientation. It should be noted that graphites possess anisotropic structures and thus exhibit or possess many properties that are highly directional e.g. thermal and electrical conductivity and fluid diffusion.
Briefly, graphites may be characterized as laminated structures of carbon, that is, structures consisting of superposed layers or laminae of carbon atoms joined together by weak van der Waals forces. In considering the graphite structure, two axes or directions are usually noted, to wit, the “c” axis or direction and the “a” axes or directions. For simplicity, the “c” axis or direction may be considered as the direction perpendicular to the carbon layers. The “a” axes or directions may be considered as the directions parallel to the carbon layers or the directions perpendicular to the “c” direction. The graphites suitable for manufacturing flexible graphite sheets possess a very high degree of orientation.
As noted above, the bonding forces holding the parallel layers of carbon atoms together are only weak van der Waals forces. Natural graphites can be treated so that the spacing between the superposed carbon layers or laminae can be appreciably opened up so as to provide a marked expansion in the direction perpendicular to the layers, that is, in the “c” direction, and thus form an expanded or intumesced graphite structure in which the laminar character of the carbon layers is substantially retained.
Graphite flake which has been greatly expanded and more particularly expanded so as to have a final thickness or “c” direction dimension which is as much as about 80 or more times the original “c” direction dimension can be formed without the use of a binder into cohesive or integrated sheets of expanded graphite, e.g. webs, papers, strips, tapes, foils, mats or the like (typically referred to as “flexible graphite”). The formation of graphite particles which have been expanded to have a final thickness or “c” dimension which is as much as about 80 times or more the original “c” direction dimension into integrated flexible sheets by compression, without the use of any binding material, is believed to be possible due to the mechanical interlocking, or cohesion, which is achieved between the voluminously expanded graphite particles.
In addition to flexibility, the sheet material, as noted above, has also been found to possess a high degree of anisotropy with respect to thermal and electrical conductivity and fluid diffusion, comparable to the natural graphite starting material due to orientation of the expanded graphite particles and graphite layers substantially parallel to the opposed faces of the sheet resulting from very high compression, e.g. roll pressing. Sheet material thus produced has excellent flexibility, good strength and a very high degree of orientation.
Briefly, the process of producing flexible, binderless anisotropic graphite sheet material, e.g. web, paper, strip, tape, foil, mat, or the like, comprises compressing or compacting under a predetermined load and in the absence of a binder, expanded graphite particles which have a “c” direction dimension which is as much as about 80 or more times that of the original particles so as to form a substantially flat, flexible, integrated graphite sheet. The expanded graphite particles that generally are worm-like or vermiform in appearance, once compressed, will maintain the compression set and alignment with the opposed major surfaces of the sheet. The density and thickness of the sheet material can be varied by controlling the degree of compression. The density of the sheet material can be within the range of from about 0.04 g/cc to about 2.0 g/cc. The flexible graphite sheet material exhibits an appreciable degree of anisotropy due to the alignment of graphite particles parallel to the major opposed, parallel surfaces of the sheet, with the degree of anisotropy increasing upon roll pressing of the sheet material to increased density. In roll pressed anisotropic sheet material, the thickness, i.e. the direction perpendicular to the opposed, parallel sheet surfaces comprises the “c” direction and the directions ranging along the length and width, i.e. along or parallel to the opposed, major surfaces comprises the “a” directions and the thermal, electrical and fluid diffusion properties of the sheet are very different, by orders of magnitude, for the “c” and “a” directions.